Why is it that there is always more to do than you have time for?

It seems to be a universal truth that you always have more to do than you have time to do it.  For instance, I keep track of the day to day through the use of the Trello app on my phone and on my laptop.  Since I have retired the number of items I have on my lists have increased while the number of lists have increased as well.  I got to the point where I started moving my lists off to a bunch of text files under the someday / maybe category (shades of my Getting Things Done days).  Even that hasn't helped.  

Then I went about creating a new set of lists based upon the Personal Kanban method.  It's a good method and I made a series of lists: Ready, Doing, Today, Done, Review Complete, and Recurring Action Items.  But even then, I am overwhelmed (even though I cant remember being whelmed).  I use the Recurring Action Items list to list out things that I do over and over again, complete with a date and time to complete.  When I finish an item on that list I set the item for the next date and time that it is due and then resort the list by due date.  That way, recurring items will always filter to the top and I can place the other items on the list in the background.  

But still, my Today list (with only three items at a time) almost never gets completed.  Why why why?